1. Field of the Invention
The present invention is generally related to nuclear system pressure vessels and in particular to the replacement of nozzle penetrations in the pressure vessels.
2. General Background
The pressurizer in a nuclear reactor coolant system establishes and maintains the reactor coolant system pressure within the prescribed limits of the system. It provides a steam surge chamber and a water reserve to accommodate reactor coolant density changes during operation. A typical pressurizer is a vertical, cylindrical vessel with replaceable electric heaters in its lower section. The heater elements extend into the vessel through nozzle or sleeve penetrations in the vessel. The pressurizer also contains a plurality of nozzle penetrations at various locations for purposes such as sensing the liquid level or temperature in the pressurizer.
Due to the operating environment, it is a common requirement that sleeve or nozzles be replaced. In some instances the entire nozzle is removed and replaced while in others only a portion of a nozzle is removed and replaced. In either case, it is necessary during the replacement process to provide a seal that prevents water seepage between the replacement nozzle and the vessel wall. The pressure vessels are normally made from a low carbon steel and water seepage would cause corrosion of the vessel wall. Since pressurizer components in nuclear power plants become radioactive after they have been in operation, it is desirable to minimize the time and amount of work required inside the pressurizer.
Applicants are aware of the following patents directed to the replacement of nozzles or sleeves.
U.S. Pat. Nos. 5,094,801 and 5,091,140 disclose an apparatus and method for replacing a heater sleeve. The original sleeve is removed and the original bore in the pressurizer is enlarged. An outer sleeve is installed in the bore with its upper end being seal welded to the cladding on the interior of the pressurizer. An inner sleeve is installed in the outer sleeve to extend into the pressurizer and is welded to the lower end of the pressurizer.
U.S. Pat. No. 5,149,490 discloses a method and apparatus for replacing a nozzle where the entire nozzle has been removed. The nozzle bore is partially tapped and a replacement nozzle is threaded therein. The end of the replacement nozzle inside the vessel is seal welded to the inside of the pressurizer. A flange on the opposite end of the replacement nozzle bears against the exterior of the pressurizer.
The known art does not address the problems associated with the need for a seal weld at the junction of the replacement nozzle and the interior of the pressure vessel.